Over time, rivers and streams continue to change course as these moving bodies of water erode the surrounding earth. One classic example of how a river may change course over time is the Colorado River and the formation of the Grand Canyon which has taken place over millions of years. Without intervention, all rivers and streams continue to change course as the force of moving water will erode surrounding earth in paths of least resistance.
When a structure is placed over a body of water, such as a bridge or culvert, it is necessary to take into account scour type erosion which will naturally take place. Such scour can occur in dramatic events such as floods, and will at least take place incrementally over time as the moving body of water continues to move particles of rock and soil downstream.
One of the most common means to ensure that a moving body of water such as a river stays its course through a built up area or at least through a manmade structure such as a bridge, is the use of a series of large boulders known as rip rap which may line the banks and/or bed of the river. Placing rip rap over an extended length of a river can be quite expensive and therefore infeasible. Such rip rap is typically quarried rock which must be transported to the work site and emplaced by heavy equipment. Due to recent environmental  awareness, placing rip rap on the banks and bed of a stream can permanently alter not only the dynamic flow characteristics of the river, but also the ecological balance of flora and fauna which may reside or depend upon an undisturbed river bed and river banks. For migrating fish populations such as salmon, rip rap can significantly alter the dynamic flow characteristics of a stream to the extent that it makes migration difficult or impossible over certain stretches of a river. Another drawback with construction which utilizes rip rap is that emplacement of the rip rap will significantly alter water quality by at least temporarily increasing the amount of sediment which is suspended in the body of water. Certain populations of fish such as trout are particularly sensitive to water quality degradation. Because of such environmental concerns, a need has developed for the ability to prevent undesirable scour which will inevitably occur within a stream or river, yet provide manmade intervention in a manner that will not significantly alter either the characteristics of the stream or river, or the surrounding vegetation and terrain which runs adjacent to the stream bank.